In general, temperatures in a spray drying chamber should be stabilized in a start-up period to avoid product overheating and to provide thermal protection of equipment downstream of the spray drying chamber. Therefore, water is usually injected through the same pressure nozzle as used for a feed liquid.
The rate of water to be sprayed must be equivalent to the water content of the feed liquid, which is usually in the range of 30-80 % by weight. Thus, the rate of water to be sprayed is also 30-80 % by weight of that of the feed liquid. Because of pressure nozzle characteristics, the spray nozzle pressure decreases to 10-80 %, depending on the liquid viscosity when this low rate of water is fed. Water droplets thus produced are likely to be so coarse that they may adhere to the surface inside the drying chamber. Subsequent spraying of the feed liquid causes dried powder to adhere to the wet surface to form deposits.
According to the prior art, devices as shown in FIGS. 6 and 7 are used to cope with this situation. In FIG. 6, a plurality of spray nozzles 10 are disposed at the top of the inside of the spray drying chamber When water is injected for spraying, the number of spray nozzles used is limited to avoid low pressure spraying. On the other hand,. FIG. 7 illustrates an example in which a water spray nozzle 11 is disposed separately from a feed liquid spray nozzle 12.
However, the device of FIG. 6 has disadvantages of uneven liquid droplet dispersion and nonuniform temperature distribution because of a relatively large distance between the nozzles and the very existence of unused spray nozzles. In addition, plugging problems are likely to occur around the nozzles left unused when water is injected.
On the other hand, the device of FIG. 7 has a disadvantage of feed liquid clogging inside the feed liquid spray nozzle since cooling or flushing cannot be conducted through the feed nozzle.
In view of the foregoing disadvantages of the prior art, it is one object of the present invention to provide a spray nozzle and a spray drying apparatus using the spray nozzle, in which water is atomized into such fine particles as to permit complete drying and is used to cool the spray nozzle as well to prevent plugging of feed liquid.